This invention relates to a control system for a batch-type pyrolysis furnace of the type used for volatilizing and burning organic material from a metal part to which the organic material is bonded. Volatiles (referred to herein as "vapor") not burned in the furnace's main chamber are burned in an incineration zone provided by an afterburner chamber in open communication with the main chamber, such communication being provided by a passage referred to as the "throat". The remaining metal part is reclaimed for reuse because the cost of reclamation is less than that of making the metal part anew. Such reclamation by pyrolysis has evolved into a subindustry of considerable economic significance not only because pyrolysis is cost-effective, but also because incineration of the vapor of polymeric materials which are not economically recyclable, conveniently and beneficially disposes of them.
A highly successful control system for a pyrolysis furnace in which incineration occurs in the main chamber and also downstream of the afterburner is provided in our U.S. Pat. No. 4,649,834. This system uses a single water spray system controlled by a first thermocouple (TC) in the main chamber (main chamber TC), a second thermocouple in the stack (stack TC) which controls only on/off or attenuated operation of the main burner and a third thermocouple (throat TC) in the vent passage ("throat") connecting the main chamber to the afterburner chamber. The effectiveness of this control system, in large measure, derives from the difference in temperatures sensed by the first and third thermocouples.
An even more effective furnace and control system is provided in our U.S. Pat. No. 4,759,298. In this system a single thermocouple (zone TC) senses the instantaneous temperature in a critical sensitive zone (CSZ) of the furnace and in cooperation with a programmable controller (PC), maintains a preselected ramp and soak temperature profile over the entire burn cycle. The CSZ was found to be within about 1 foot from the upper edge of the throat, and lower than about 6" (inches) from the ceiling of the main chamber. When the temperature required by the profile was exceeded, a single water spray actuated by a signal from the PC lowered the temperature below the preset profile. The zone TC thus maintains a fire under controlled temperature conditions in the main chamber without an explosion, using a single TC in the control system.
The '298 pyrolysis furnace, like the one of this invention, generated a characteristic smokeless discharge into the atmosphere under normal conditions of commercial operation. By "discharge" we refer to combustion products issuing from the furnace's stack, and by "smokeless" we refer to the discharge being substantially clear to the naked eye, that is, permeable to light in the visible wavelength range. The problem was that, when burning solvent-rich paints and volatile hydrocarbon rubbers, occasionally, there were explosions.
We stated in our '298 patent that "The required ramp may consist of a single ramp, or plural ramps, and the one or more ramps may be executed with no soak periods, if a soak period is unnecessary, or plural soak periods." We preferred 4 ramps and 4 soak periods, and as in our '834 patent, shut off the main burner when the water spray was actuated to lower the temperature when it rose above the desired profile, as this was the rational thing to do to lower temperature.
We typically used a ramp at more than 2.degree. F. per minute, and did not recognize the critical effect that the rate of ramping had on control of the burn, despite controlling the ramp with one or more water sprays, because we switched off the main burner to help control rising temperature. Though switching off the main burner effectively lowered the temperature, we did not realize the burning load was still hot enough to ignite vapors building up in the main chamber explosively. This was not a problem as long as the main chamber was sealed against leakage of air (oxygen) into it. But we found that it was not practical to seal the main chamber in such a way as to prevent explosions. With the foregoing '298 system, we found we had to confine the single TC to the CSZ in the main chamber. We did not know the criticality of the location of the TC was related to the rate of ramp. It simply did not occur to us that we should lower the temperature without shutting off the main burner during a "burn" while at the same time keeping the water spray ON, for the sole purpose of having an excess of oxygen in the main chamber at all times. If there is an excess of oxygen at all times during the burn, we found that there is no sudden buildup of volatiles, hence no explosion.
We accidentally discovered that we could lower the temperature as just stated, if we staged the main burner while it stayed ON. We could keep the main burner on "high-fire", that is, burning a full, normal, fuel supply, which did not permit the build-up of volatiles in the main chamber, and despite being on "high-fire" we could simultaneously decrease the temperature, provided we sprayed enough water on the load. Further, though keeping the main burner ON seemed to be the wrong thing to do to lower the temperature in the main chamber, we found that, if the steps of the ramp were small enough (hence "micro-ramped"), we could get to the desired set-point temperature where the main burn was to occur, while keeping the main burner on high-fire and maintaining a primary water spray for a substantial portion of the time it took to ramp the temperature to the set-point.
Moreover, the ramp being controlled only by a preselected microstep of no more than 2.degree. F./min, preferably less, the gradient of the ramp depended on the amount of heat generated per unit weight of organic burnables per unit of time, referred to as the "heat generation factor" ("HGF" for brevity). The HGF for various burnables is evaluated on a relative scale, the HGF for a specific butyl rubber such as is used in automotive motor mounts being 1, on a scale of 10. Metal fixtures coated with asphalt or enamel paint have an HGF of about 5; with lacquer or epoxy paint, about 6; with polyurethane, about 7 to 8; and, with nitrocellulose paint about 9. Knowing the general nature of the burnables and the HGF, the time for the main burn would be set by trial and error, once the set-point was reached.
This invention is specifically directed to obviating explosions when burning highly combustible, relatively large loads of metal parts combined with silicone-free polymers ("burnables") which are to be incinerated without an explosion, and smokelessly, in a relatively small main chamber, that is, with a relatively high ratio of load (lb)/volume (ft.sup.3), referred to as the load/volume ratio. Such loads contain from 0.1 lb of burnables per lb of metal, to 2 lb burnables/lb metal, and are referred to as "high-polymer" loads in contrast to conventional loads which contain less than 0.1 lb burnables/lb of metal.
The term "pyrolysis oven" has been used in the art, by others, to indicate that there is no incineration of organic material on the metal parts within the oven's main chamber. The material is simply volatilized (or vaporized) without being burned in the oven's main chamber. The vapors are then burned in the afterburner chamber, but not before they have exercised the opportunity to plug water spray nozzles used to keep the volatilization of burnables in the main chamber under control. Such operation of a "pyrolysis oven", where there is no fire in the main chamber, is supposed to clearly distinguish its function, from that of a "pyrolysis furnace" in which there is. Nevertheless, the terms are often misused or interchanged, particularly in relation to devices using an afterburner in an afterburner chamber of the furnace, with no thought given as to the significance of where the fire is maintained.
It is now well-known that with high-polymer loads with the above-specified burnables content, the rise of temperature in the initial portion of the burn cycle was often uncontrollable, resulting in dense smoke and excessive temperatures in the main chamber. This occurred even when the furnace is constructed with a "vent number" greater than 0.003/ft found to be critical for normal operation. The vent number is computed by dividing the area of the vent (throat, ft.sup.2) by the volume of the main chamber (ft.sup.3).
The vapor to be incinerated is generated when mounting means for engines and electric motors (collectively referred to as "motor mounts"), and similar steel parts bonded to rubber; or, copper-containing electrical parts such as armatures, stators, transformers and the like; or, painted ferrous or non-ferrous steel parts; or, metallic bodies of arbitrary shape which are coated with, or bonded to polymeric materials (referred to herein as "polymer-bonded metal parts"), are to be pyrolized in a pyrolysis furnace.
Polymeric materials to be disassociated from metal parts are such materials as are commonly bonded to a metal substrate or matrix and include natural and synthetic elastomers; for example, natural rubber and synthetic rubber which are polymers of dienes; silicones which are polymers of siloxanes and the like; and, natural and synthetic resinous materials including natural shellac and synthetic plastics such as phenolics and acrylics, particularly paints. The difficulty of incinerating the materials smokelessly varies; silicones do not burn smokelessly, but silicone-free rubbers and paints can now be more reliably and economically incinerated, and smokelessly.
The foregoing polymeric materials are to be separated from the metal matrix to which they are bonded without melting the metal, and preferably, in most instances, without causing warpage or other undesirable deformation of residual metal matrix. It is self-evident that such separation may be effected by directly incinerating the polymeric materials, as is typically done in an incinerator for waste, but it is equally self-evident that the requirement of incineration without damaging the metal parts will not be met. Of course, damage to the parts can be minimized if only a few parts are incinerated together, but this method is undesirable because it does not lend itself to reclaiming a large enough mass of parts to be economical.
More so than the desirability of a smokeless discharge from the stack of a pyrolysis furnace, the desirablility of burning highly combustible burnables without an explosion cannot be overemphasized. It is common practice to operate such a furnace during the day in such a manner that the smoky discharge is not too objectionable, reserving such operation for darkness More responsible operators provide plural afterburners in series to make sure that as complete combustion as possible, is obtained. The seriousness of the problem is such that even in a drying furnace where a relatively small amount of contaminating oil is being burned, plural burners are used, as disclosed in U.S. Pat. Nos. 3,767,179 and 3,839,086 to Larson. The seriousness of explosions is measured in more dismal statistics.
Where the weight ratio (weight of burnables to be burned): (weight of metal) is relatively high, that is in the range from 0.1:1 to 2:1, a manufacturer of a prior art furnace advises against burning such loads. Attempts to burn even a small load result not only in the discharge of a highly noticeable stack gas, but also in the severe fouling of the furnace's main chamber, the controls, and, most important, of the water nozzles upon which the safe operation of the furnace is critically dependent.
An attempt to deal with the problem of fouling water spray nozzles is found in U.S. Pat. No. 4,557,203 to Mainord who uses a first sensor in the stack downstream of the afterburner to actuate a first set of nozzles; and a second sensor in the main chamber to actuate a second set of nozzles.
This invention is specifically directed to a pyrolysis furnace with a single thermocouple, located anywhere in the main chamber provided the temperature is "micro-ramped"to preselected progressively higher set-points at a ramp not to exceed 2.degree. F./min, preferably about 1.degree. F./min, or less, the higher the HGF of the load, the lower the ramp rate. Moreover, a micro-ramp requires no initial soak period to bring the load to ignition temperature, no intervening soak intervals, and the main burner is never shut off during the entire burn period, that is, even after the a major portion, preferably at least 80 percent by weight (% by wt), and most preferably about 90% by wt of the burnables are burned. After the major portion of burnables is burned, the temperature is maintained constant during a final burn (referred to as the "final soak period") effected with the main burner being starved of fuel (at "low-fire"). The surprising result is that there is no explosion because there is never a runaway increase of temperature.
It is known that heating of the metal parts to 700.degree.-800.degree. F. in an enclosure with limited air intake will char or degrade all known combustible contaminants without ignition if the percentage of contaminants is less than about 2% by weight ("wt") of the parts. However, we are concerned with igniting much higher amounts of combustibles in the range from about 10% by wt of the load in the charge to about twice the weight of the load, or even more, and it is critical that the burning of the combustible not result in an explosion, yet produce an essentially smokeless stack.
It is unnecessary to point out that, when operating with highly flammable loads, under near-explosive conditions, a very small misstep can set off an explosion. The inability to control the burn when the ramp is set for more than 2.degree. F./min is such a misstep. Our invention avoids such a misstep.